152 



THE FOOD OF ANIMALS 



PoLYCLADES are distinguished by the extreme thinness of the 

 oval or leaf-shaped body, which is often so closely pressed against 



a stone or other firm body as to be difificult 

 to detach without tearing. The feeding 

 habits have to some extent been observed 

 in certain cases. The common British 

 species Leptoplana tremellaris, for example, 

 includes marine bristle-worms among its 

 prey, enveloping them within its pharynx, 

 which, when protruded, assumes the form 

 of a frilled funnel (see vol. i, p. 446). It 

 is suggested that the peculiar rodlets which 

 can be ejected from the skin have previ- 

 ously been used to overpower the victim, 

 and that weakly specimens are most liable 

 to attack. Once within the pharynx a 

 powerful digestive juice is poured out upon 

 the food, which in a softened condition is 

 drawn back into the stomach for complete 

 digestion. One form [Prosthiostomum), in 

 which the mouth is placed far forwards, 

 possesses a long tubular pharynx which can 

 be rapidly protruded, proboscis fashion, and 

 used to secure small worms. 



More numerous observations have been 

 made upon the Triclades (fig. 409), which 

 are highly carnivorous and, as in the pre- 

 ceding group, use the pharynx as a weapon 

 of offence, pushing it out from the centrally 

 or, it may be, backwardly situated mouth. 

 There is little doubt that in this group also 

 the rodlets shot out from the skin exert a 

 paralysing influence upon the prey. The freshwater specimens are 

 known to attack worms, water-snails, and water- beetles, among other 

 invertebrate forms. Land Planarians, which belong to this group, 

 feed to a very large extent upon earth-worms, some of them being 

 adapted to pursue these creatures underground, capturing them 

 with the help of a sticky fluid poured out upon the under surface. 



Some, at least, among the Rhabdoccele species are carnivorous 

 in habit, and certain marine species are parasitic. 



Fig. 409.— A Triclade Planarian (much 

 enlarged),with pharynx protruded. The 

 branched gut is seen within the body, j 



